1. Field of the Invention
The present invention relates to a projector for projecting a video image onto a screen and, more particularly, to a technique for suppressing unnecessary heat generation by a power supply circuit in a standby mode.
2. Description of the Related Art
In a casing of a projector, heat sources such as a lamp and a power supply circuit are provided. When the temperature in the casing becomes high due to heat generated from these heat sources, various adverse effects such as thermal deformation of the casing and deterioration in the performances of electric circuits occur. In order to dissipate the heat generated from the heat sources to outside, a cooling fan is provided. Generally, a projector has the function of the standby mode of turning off a lamp. By setting the standby mode except when the projector is used, the life of the lamp is extended and electricity can be saved. Japanese Patent Laid-Open (JP-A) No. 2004-361466 discloses a projector realizing reduction in noise by stopping the rotation of a cooling fan in the standby mode.
On the other hand, as a power supply circuit of a projector, a circuit obtained by adding a power-factor improving circuit to a switching regulator is generally used. The switching regulator is a stabilized DC power supply for performing DC-DC conversion by turning on/off a switching element connected to a transformer at high speed. The power-factor improving circuit is provided to improve power consumption efficiency by improving the power factor seen from the power supply side by making a current waveform close to a voltage waveform. JP-A No. Hei 11-308858 discloses a power supply circuit having a switching regulator and a power-factor improving circuit (in this document, however, a projector is not described).
In a conventional projector, the power-factor improving circuit operates not only in a normal operation mode in which the lamp is ON but also in the standby mode in which the lamp is OFF. An inherent purpose of the power-factor improving circuit is, however, to prevent occurrence of noise failure. The noise failure occurs when a current waveform becomes a pulse shape even if a voltage waveform is a sine wave in a state where the lamp is ON, the power factor decreases and, due to the decrease, higher harmonics enter a power supply line. In the standby mode in which the lamp is OFF, current of higher harmonics is not an issue. Therefore, it is useless to make the power-factor improving circuit operate in the standby mode. In addition, heat is generated by the power supply circuit, so that forced cooling with a fan becomes necessary. As a result, power consumption increases and a problem of noise of the fan occurs.
In the above-mentioned JP-A No. Hei 11-308858, in a pre-heat (standby) state in which power supply to a load is stopped, the operation of the power-factor improving circuit is stopped by a control signal output from an ON/OFF control unit. With the configuration, the power-factor improving circuit does not operate in the standby mode, and unnecessary heat generation can be suppressed. However, as mentioned above, JP-A No. Hei 11-308858 does not include description of a projector. According to JP-A No. Hei 11-308858, concrete technical means used for stopping the operation of the power-factor improving circuit in the projector is not clear. For example, it may be assumed that when the standby mode is set by an operation unit of the projector, a signal indicative of the standby mode is generated by a microcomputer and sent to a power supply circuit, thereby stopping the operation of the power-factor improving circuit. However, in this case, software for generating and transmitting a special signal and a signal line extending from a control board to a power supply board are additionally required, which causes a problem of an increase in the cost accordingly. Neither does JP-A No. 2004-361466 describe a solution to such a point.